Modern water skis can be laminates of plastics and other materials. For example, Hancock et al. U.S. Pat. No. 4,731,038, issued Mar. 15, 1988, discloses a water ski having a "preformed core" consisting of a wide top "outer deck" joined to a narrower and lower "inner plate" by a thin intervening layer of rigid plastic foam or honeycomb material. Preferably the outer deck and inner plate are aluminum, although they can be plastic. The composite core can be formed by placing the inner plate in the bottom of the cavity of a mold, laying the outer deck on the mold so that it bridges across the mold cavity, filling the space between the deck and the plate with "reaction injected molding material" and allowing such material to set or cure. The composite core is removed from the mold and placed with the laterally projecting lips of the outer deck resting on the rim of a mold having a larger cavity which is shaped to have the desired contour of the bottom and sides of the finished ski. The inner plate and foam layer are located within the mold cavity, spaced inward from the sides of the cavity and a substantial distance above the bottom. The mold is closed and a softer foam is injected to surround the inner plate and upper rigid foam layer below the outer deck. The softer foam forms the "body" of the ski, i.e., the bottom and the side portions below the lip of the top deck. After removal from the second mold, the top deck is trimmed flush with the sides of the softer ski body to complete construction of the ski, although a "finish coating" can be added if desired.
Sheurer et al. U.S. Pat. No. 4,897,063, issued Jan. 30, 1990, discloses a plastic ski having an internal reinforcing skeleton of steel rods positioned by plastic clips. Such skeleton is placed in a mold and polyurethane foam is injected and allowed to set or cure to form the ski. A preformed reinforcing bottom sheet or preformed reinforcing top sheet can be placed in the mold prior to injection of polyurethane foam to form one finish surface of the ski.
Weinhaus U.S. Pat. No. 4,386,982, issued Jun. 7, 1983, discloses another laminated water ski. In the case of the Weinhaus construction, top and bottom preformed shells have interlocking sides. The space between them is filled with "a predetermined quantity of liquid formable resin" and allowed to cure to form the composite ski.
Another known method of manufacturing a laminated ski is to form a thin bottom shell having the desired contour of the bottom and sides of the finished ski and to mold separately a rigid polyurethane core having substantially the desired shape of the ski. The core is wrapped with one or more mats, tapes or fabrics of fibrous reinforcement and coated with thermosetting epoxy resin. The shell is placed in a mold having a cavity of the same contour as the shell, followed by insertion of the wrapped and epoxy-coated core. The coated core is placed inside the shell and then is covered by a top plastic deck sheet. The mold is closed and sufficient heat and pressure are applied to set the resin for bonding the layers together. However, since the preformed polyurethane core typically has a slick, smooth outer surface, it is necessary to abrade the outer surface of the core, such as by sanding, sand blasting or another manner of etching or roughening in order to achieve a firm bond between the core and the top deck and bottom shell.
Peerlkamp U.S. Pat. No. 4,457,729, issued Jul. 3, 1984, is concerned with the problem of bonding polyolefin plastics to a substrate or core. For example, it is stated in the paragraph beginning at column 1, line 19:
To adhere polyethylene to a substrate, an adhesive must be used, or the surface of the polyethylene or the substrate must be etched. However, adhesives effect only a limited degree of adhesion due to the poor bonding properties of polyethylene.
The solution proposed by Peerlkamp was to rotomold an outer layer of "stabilized polyolefin" and, while the outer layer remains in the mold, cover the interior of the outer layer with an "unstabilized or hardly stabilized" polyolefin, prior to introducing a foaming plastic such as polyurethane into the interior of the mold. Peerlkamp states that this results in very good adhesion between the polyurethane foam and the unstabilized or hardly stabilized polyethylene, and between the unstabilized or hardly stabilized polyethylene and the outer layer of polyethylene.
In addition to manufacturing complexities and difficulty in bonding layers of laminated water skis, in the past it has been difficult to vary factors that affect performance of the ski independently of other performance-affecting factors. Such factors include the outline shape of the ski, bottom contour, edge profile, weight of the ski and its stiffness. Conventionally, stiffness is varied by changing the materials used in the manufacture of the ski, which may require a change in the manufacturing process, or by increasing the thickness of the ski. Changing materials or ski thickness can affect the weight of the ski, and changing thickness also affects the edge profile. Thus, in the conventional method, stiffness cannot be varied without affecting one or more other performance characteristics.